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Sheave re-size verification

I just want to run this by someone.... The new sheave I have to get is going to be around $450 after everything is said and done...

Unit is a Trane M series MCCB040UA CHW air handler... Constant Volume. Spec'd for 17,000 CFM (approx). Unit is actually putting out 22,000 CFM. I figure it to be 135% over spec. (Airflow was measured by duct traverse using a 48" AirFoil and ADM870c Airdata Multimeter.)

Motor sheave is a 5.30", fan sheave is a 14". Since a 4" motor sheave is not available in the configuration I need, I requested a 19" fan sheave. (Sheaves are 4 groove, 3VX type, with QD bushings.) Motor speed is 1760, 25HP.

I figure the new sheave size by taking the size and multiplying by 1.35 to get the 19" sheave (comes out to 18.9). I had figured the original motor sheave by taking 1.35 1/x which gives me .74.... .74 * 5.3 = 3.9

So, am I right, or did I just make a $450 mistake? Have to do this sheave change because the unit is over air, and the returns are rattling from the airflow. (This unit serves a gym and a couple of small rooms....)

put a vfd on motor and forget it. Original design was for a specific static presure and with minimal ducting you probably are under that pressure. It would be difficult to know what you are operating on and you could vary speed w/ cooling stages.